Cellular detection and identification are proving vital for the prevention and diagnosis of many diseases related to cancer and both viral and bacterial infections. Techniques for cellular detection can be divided into two main categories, optical and electrical. Optical techniques rely on fluorescence of dyes attached to targeted cells. The dyes emit light when stimulated, allowing for detection through an optical microscope. This technique has proven versatile in detecting small quantities of the targeted cells. However, the use of optical components can be expensive. In addition, the need of mixing the appropriate dyes limits the number of various types of cells that can be identified simultaneously. Also the dyes themselves can be toxic to the cells through a process called phototoxicity.
Many electrical based techniques operate by measuring the change in impedance due to the presence of the target cells. These systems incorporate electrodes that measure the impedance or current between them. Currently there are two mechanisms used for electrical detection and identification of cells. The first method relies on functionalizing electrodes with a bioreceptor, which is designed to be highly selective. These electrodes are immersed into a solution containing the cells. The bioreceptors will bind to the targeted cell, immobilizing them on top of the electrodes. The immobilized cell will change the measured impedance or current either by insulating them or by changing the conductance of the fluid around the electrodes due to an increase in ions that surround the cells.
Microfluidic devices use a much smaller volume of cell suspension than the immersion technique. Electrodes or the walls of the fluidic channel can be functionalized to immobilize the targeted cells. The cell suspension is pumped through the channel and over the electrodes. The functionalized surface would capture cells as they flow over it, reducing the overall volume of suspension that is flowing between the electrodes. This will result in a change in the measured impedance or current between the electrodes.
Another electrical based technique relies on functionalizing magnetic beads with the bioreceptors instead of the electrodes or walls. The beads are mixed with the sample, allowing the targeted cells to attach to them. After which the targeted cells are separated from the suspension by using a magnetic field. The cell-coated beads are then re-suspended in a low conductive media and either placed or flown over electrodes. The entire device is placed in a magnetic field, such that the beads are attracted to the electrode surface. The resulting accumulation of cell-coated beads lowers the impedance to indicate the presence of the targeted cells. Without the cells the drop in impedance is much smaller. This method has an advantage of having a higher sensitivity over the functionalized electrodes or walls since the magnetic beads can better cover the electrodes. However, both techniques still lack the ability to simultaneously detect a larger variety of cells in a single sample, which is required for a rapid general diagnosis.